1. Field of the Art
The present invention relates to an apparatus for reading out or retrieving information from a magneto-optical storage medium.
2. Description of the Prior Art
There is known an apparatus for reading out, retrieving or reproducing, through a magneto-optical effect, information which has been written on a storage medium of a magneto-optical material having a self-magnetizing property and a comparatively high transmittance (transmission factor) of light, the information being written in individual memory locations of the storage medium through local magnetizations thereof in one of opposite directions normal to the surface of the medium. Generally, such an apparatus reads out the stored information from the magneto-optical storage material, by utilizing the Kerr effect or Faraday effect, wherein when linearly or plane polarized light is incident upon the magneto-optical material with the incident path parallel to the direction of magnetization of the material, the plane of wave vibrations (polarization) of the light reflected from or transmitted through the magneto-optical material are rotated in relation to the direction in which a given memory location is magnetized for data recording.
In the known apparatus, however, a half mirror or semi-transparent mirror is commonly used to separate the incident light and the reflected light, and the use of such a half mirror causes a loss of light, which is a drawback of the traditional apparatus. Further, the known apparatus requires an analyzer or analyzers (analyzing device) for detecting the linearly polarized light whose plane of vibrations is rotated by the Kerr or Faraday effect. Since the angle of rotation of the plane of vibrations of the linearly polarized light due to the Kerr or Faraday effect is extremely small (0.3 to 0.5 degree approximately), the apparatus requires a differential detecting device or the like for exact detection of such a minute variation in the linearly polarized light transmitted through the analyzer The requirements for such analyzer(s) and differential detecting device make the apparatus complicated in structure and consequently lead to increased cost of manufacture.
In the art of such a magneto-optical data reading or retrieving apparatus, a laser or monochroic light source is commonly used. These light sources are subject to variation in output (quantity of light produced), and the output variation is inevitably introduced as noises into a detected angle of rotation of the plane of vibrations, or detected quantity, of the light reflected from or transmitted through a magneto-optical storage medium Further, a difference in the above rotating angle or quantity of the light due to change in the magnetizing direction of the magneto-optical material is relatively very small. Therefore, the signal obtained in the traditional apparatus to read out the written information from the magneto-optical material does not yield a satisfactory high S/N ratio for accurate reading of the stored information.